In a typical cellular system, also referred to as a wireless communications network, wireless terminals, also known as Mobile Stations (MS) and/or User Equipment units (UEs) communicate via Radio Access Networks (RAN) to a core network. The wireless terminals may be mobile stations or user equipments such as mobile telephones also known as cellular telephones, and laptops with wireless capability, e.g., mobile termination, and thus may be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being controlled by a base station, e.g. a Radio Base Station (RBS), which in some radio access networks is also called eNodeB (eNB), NodeB or B node. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
In some versions of the radio access network, several base stations are typically connected, e.g. by landlines or microwave, to a Radio Network Controller (RNC), as in Third Generation (3G), i.e. Wideband Code Division Multiple Access (WCDMA). The radio network controller supervises and coordinates various activities of the plural base stations connected thereto. In Second Generation (2G), i.e. Global System for Mobile communication (GSM), the base stations are connected to a Base Station Controller (BSC). The network controllers are typically connected to one or more core networks.
In an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) radio access network the creation of neighbor relations to neighbor nodes is to a large extent automated by the Automatic Neighbor Relation (ANR) feature. ANR is a feature aiming to relieve the operator from needing to manually configure neighbor cell lists and associations, i.e. neighbor relation lists. It is currently only standardized in E-UTRAN eNodeBs and user equipments, but may be used to retrieve information about E-UTRAN, WCDMA and GSM neighbor cells.
FIG. 1, from a Third Generation Partnership Project (3GPP) Technical Specification (TS), shows the main principle of ANR in the case a new neighboring Long Term Evolution (LTE)/System Architecture Evolution (SAE) cell is detected. An eNB 101 controlling a source cell A has an ANR function. As a part of the normal procedures, the eNB 101 instructs a user equipment 105 to perform measurements on neighbor cells, e.g. cell B. The frequency of the measurements on neighbor cells depends on the logic implemented in the eNB 101, and may for example be periodical. The source cell A has Physical Cell ID (Phy CID)=3, and the cell B has Phy CID=5. The function works as follows:
Step 1
The user equipment 105 sends a measurement report regarding cell B to the eNB 101. This report comprises Cell B's Physical Cell ID (PCI), but not it's E-UTRAN Cell Global Identifier (ECGI). The report may comprise for example Phy-CID=5 and information that the signal from the cell B is strong.
Step 2
The eNB 101 instructs the user equipment 105, using the newly discovered PCI as parameter, to read the ECGI, indicated as Global-CID in FIG. 1, the Tracking Area Code (TAC) and all available Public Land Mobile Network Identity PLMN ID(s) of the related neighbor cell. To do so, the eNB 101 may need to schedule appropriate idle periods to allow the use equipment 105 to read the ECGI and the other information from the broadcast channel of the detected neighbor cell.
Step 3
The user equipment 105 reads system control information at cell B on the downlink Broadcast Control Channel (BCCH).
Step 4
When the user equipment 105 has obtained the ECGI of cell B, the user equipment 105 reports the detected ECGI to the eNB 101 controlling the source cell A. The source cell may also be called serving cell. In addition the user equipment 105 reports the TAC and all PLMN IDs that have been detected to the eNB 101.
Step 5 (Not Shown)
The eNB 101 decides to add this neighbor cell B to the neighbor relation list, and may use PCI and ECGI to update the neighbor relation list. If needed, it may setup a new X2 interface between eNB 101 and the new eNB 103. An X2 interface is the interface between two eNBs which supports exchange of signaling information between the eNBs.
This automatic addition of neighbor cells means that potentially “bad” neighbor cells are added and used as handover candidates which may decrease the handover performance, and which leads to an increased number of handovers.
The size of the source cell is configured into an eNB, this attribute is specified in 3GPP as the parameter cellSize providing the size of the cell coverage area of the source eNB. In 3GPP the parameter cellSize is specified with the values verysmall, small, medium, large. The information about cell size is only used in conjunction with a feature called “UE history” with the intention to detect toggling between cells, i.e. a eNB comprises this information when handing over a UE/session to another eNB. The UE history comprises information about cells that a UE has been controlled by in active state prior to the target cell.